The early evolution of birds involves a step where the long tail of reptilian predecessors
is reduced and forms a pygostyle at the end where in extant birds tail feathers
(rectrices) insert. Whereas the tail of bipedal theropods could help to keep balance,
tail feathers of birds serve flight manœuvres. In addition to a pygostyle avian predecessors
developped a synsacrum typical of modern birds. In extant birds this synsacrum houses
specializations which seem to function as a sense organ of equilibrium. I am interested
in the question whether avian predecessors with a pygostyle and a synsacrum do have
these specializations (see page Evolution)